Peter Holtappels

Reaction of CO/CO2 gas mixtures on Ni–YSZ cermet electrodes

The reaction of carbon monoxide/carbon dioxide mixtures on Ni–YSZ cermet electrodes was investigated as a function of the electrode potential and the partial pressures of the reactants at 1273 K. Time-dependent reaction rates are observed for the CO oxidation reaction for oxygen activities corresponding to open circuit potentials in the range from −750 to −1010 mV. The electrode changes between a passive state and several active states for the CO/CO2 reaction. Periodic changes of the reaction rate for the CO oxidation are observed every 30 and 80 s. The impedance spectra recorded at the rest potential and the overpotential dependence of the CO oxidation rate indicate a change in the number of active sites in the reaction zone. In the active state, the CO oxidation reaction is more than one order of magnitude slower than the hydrogen oxidation reaction on these Ni–YSZ cermet electrodes. These results indicate clear differences in the kinetics of the CO and H2 oxidation reaction.

Durability test of SOFC cathodes

The durability of solid oxide fuel cell (SOFC) composite cathodes of lanthanum strontium manganite and yttria stabilised zirconia was investigated. The cathodes were kept at constant, realistic operating conditions (−300 mA cm−2 at 1000 °C in air) for up to 2000 h. After the 2000 h test the increase in electrode overvoltage exceeded 100% of the initial value. Nominally identical cathodes kept for 2000 h at 1000 °C in air without current load for comparison showed little or no degradation. Thus, the current load of −300 mA cm−2, rather than the operation temperature of 1000 °C, was responsible for the degradation. Structural analysis showed an increase in the porosity at the electrode interfaces, when the electrode had been polarised. No such structural changes were found for electrodes tested without current load. The degradation is primarily ascribed to pore formation in the electrode material induced by an electric field.

Electrical conductivities and chemical stabilities of mixed conducting pyrochlores for SOFC applications

Pyrochlores with praseodymium as the A-site cation and zirconium, tin, cerium and manganese cations on the B-site were prepared in air and their electrical conductivities were investigated as a function of oxygen partial pressure and temperature. Pure Pr2Zr2O7±δ as well as samples modified on the B-site with 5% Mn or 20% Ce show conductivities, which are lower than 2×10−3 S/cm at 1000°C in H2/H2O atmospheres. Electronic p-type conductivity was indicated for these materials in oxygen/nitrogen mixtures. The electrically conducting pyrochlore solid solutions Gd2TiMoO7±δ and Gd2Ti0.6Mo1.4O7±δ were synthesised and investigated in 1% H2/3% H2O/96% N2. No formation of a new phase by reaction with YSZ was indicated after exposure to this atmosphere at 1000°C for 1000 h. Pr2Sn2O7±δ modified with 5% indium on the B-site exhibited a conductivity in air of 6.5×10−3 S/cm at 1000°C. The pure material was found to decompose in mixtures of 9% H2/3% H2O/88% N2.

Electrochemical Characterization of Ceramic SOFC Anodes

A simple method for the rapid electrochemical characterization of solid oxide fuel cell (SOFC) anodes is presented. Current-potential and impedance measurements have been performed at operating temperature in a simplified manner using unsintered ceramic electrodes. Results for materials with different crystal structures and electrical properties are presented and show characteristic differences attributed to the materials electrical properties. At 1000°C in hydrogen saturated with water at room temperature materials with dominant electronic conduction show polarization resistances in the range 10-13 Ω cm 2 . For mixed conducting ceramies as Y 0.2 Ti 0.18 Zr 0.62 O 1.9±δ (YZT) and Ce 0.6 Gd 0.4 O 2.δ the electrochemical performance is improved to 3.7 and 0.44 Ω cm 2 , respectively. The best value measured for YZT was 0.7 Ω cm 2 when sintering the layer at 1300°C. Initial evaluation of the impedance spectra indicates an extension of the reaction zone into the porous mixed conducting electrode, which can explain the higher electrochemical activity of the mixed conductors Ce 0.6 Gd 0.4 O 1.8.δ and YZT.

Modified strontium titanates: from defect chemistry to SOFC anodes

Modified strontium titanates have received much attention recently for their potential as anode material in solid oxide fuel cells (SOFC). Their inherent redox stability and superior tolerance to sulphur poisoning and coking as compared to Ni based cermet anodes could improve durability of SOFC systems dramatically. Various substitution strategies can be deployed to optimise materials properties in these strontium titanates, such as electronic conductivity, electrocatalytic activity, chemical stability and sinterability, and thus mechanical strength. Substitution strategies not only cover choice and amount of substituent, but also perovskite defect chemistry, distinguishing between A-site deficiency (A1−xBO3) and cation-stoichiometry (ABO3+δ). Literature suggests distinct differences in the materials properties between the latter two compositional approaches. After discussing the defect chemistry of modified strontium titanates, this paper reviews three different A-site deficient donor (La, Y, Nb) substituted strontium titanates for their electrical behaviour and fuel cell performance. Promising performances in both electrolyte as well as anode supported cell designs have been obtained, when using hydrogen as fuel. Performances are retained after numerous redox cycles. Long term stability in sulphur and carbon containing fuels still needs to be explored in greater detail.

Electrical conductivity of titanium pyrophosphate between 100 and 400 °C: effect of sintering temperature and phosphorus content

The synthesis of titanium pyrophosphate is carried out, and the material is sintered at different temperatures between 370 and 970xa0°C. Yttrium is added during the synthesis to act as acceptor dopant, but it is mainly present in the material in secondary phases. The conductivity is studied systematically as a function of sintering temperature, pH2O, pO2, and temperature (100–400xa0°C). Loss of phosphorus upon sintering above 580–600xa0°C is confirmed by energy dispersive spectroscopy and combined thermogravimetry and mass spectrometry. The conductivity decreases with increasing sintering temperature and decreasing phosphorus content. The highest conductivity is 5.3u2009×u200910−4xa0Sxa0cm−1 at 140xa0°C in wet air (pH2Ou2009=u20090.22xa0atm) after sintering at 370xa0°C. The conductivity is higher in wet atmospheres than in dry atmospheres. The proton conduction mechanism is discussed, and the conductivity is attributed to an amorphous secondary phase at the grain boundaries, associated with the presence of excess phosphorus in the samples. A contribution to the conductivity by point defects in the bulk may explain the conductivity trend in dry air and the difference in conductivity between oxidizing and reducing atmospheres at 300–390xa0°C. Slow loss of phosphorus by evaporation over time and changes in the distribution of the amorphous phase during testing are suggested as causes of conductivity degradation above 220xa0°C.

Real-SOFC – A Joint European Effort in Understanding SOFC Degradation

The Integrated Project Real-SOFC joins 26 partners from throughout Europe active in planar SOFC technology. The project is funded by the European Commission within the Sixth Framework Programme and aims at improving the understanding of degradation in SOFC stacks and extending the durability of planar SOFC devices such that they become viable for stationary applications. The project consortium comprises of universities, research institutions and industrial companies. The project concept is based on improving materials and materials processing by the provision of extensive test results allowing the identification of degradation mechanisms, and then to supply industrial components of enhanced quality for repeated testing analysis. An iterative programme of component development is employed resulting in the production of ‘2nd’ and ‘3rd’ generations of SOFC components/stacks. This paper offers some recent results on the degradation issues and covers selected aspects of other project-related output such as testing conditions, environmental impact assessment and educational activities.

Charge Localization in the Lithium Iron Phosphate Li3Fe2(PO4)3 at High Voltages in Lithium-Ion Batteries

Possible changes in the oxidation state of the oxygen ion in the lithium iron phosphate Li3Fe2(PO4)3 at high voltages in lithium-ion (Li-ion) batteries are studied using experimental and computational analysis. Results obtained from synchrotron-based hard X-ray photoelectron spectroscopy and density functional theory (DFT) show that the oxidation state of O(2-) ions is altered to higher oxidation states (O(δ-), δ<2) upon charging Li3Fe2(PO4)3 to 4.7u2005V.

Ni-Based Solid Oxide Cell Electrodes

This paper is a critical review of the literature on nickel-based electrodes for application in solid oxide cells at temperature from 500 to 1000 °C. The applications may be fuel cells or electrolyser cells. The reviewed literature is that of experimental results on both model electrodes and practical composite cermet electrodes. A substantially longer three-phase boundary (TPB) can be obtained per unit area of cell in such a composite of nickel and electrolyte material, provided that two interwoven solid networks of the two solid and one gaseous phases are obtained to provide a three - dimensional TPB throughout the electrode volume. Variables that are used for controlling the properties of Ni-cermet electrodes are: (1) Ni/YSZ volume ratio, and (2) porosity and particle size distribution, which mainly affected by raw materials morphology, application methods and production parameters such as milling and sintering. The various electrode properties are deeply related to these parameters, but also much related to the atomic scale structure of the Ni-electrolyte interface, which in turn is affected by segregation of electrolyte components and impurities as well as poisons in the gas phase. The main emphasis will be on the following subjects: (a) electronic conductivity of cermets, (b) dimensional and thermodynamic stability including redox cycling, (c) thermal expansion coefficient matching, (d) chemical compatibility with stack components and gaseous reactants and (e) electrode reaction mechanism and polarisation resistance. A brief discussion of the main concepts in the modelling literature is given in context of the latter subject.

Conductivity at Low Humidity of Materials Derived from Ferroxane Particles

) is measured in air (pH2O =0.037 atm) at room temperature on sintered material. The conductivity values are compared with other works in the literature and the dependence of conductivity on surface area and pore size is discussed. It is suggested that both unsintered and sintered materials act as proton conductors at room temperature under moderate humidity conditions.